1. Field of the Invention
The present invention relates to an ultrasonic imaging apparatus for directing an ultrasonic beam to a subject under examination, converting an echo wave from the subject to an electric signal, or an echo signal, and processing the echo signal for displaying an image.
2. Description of the Related Art
In a conventional ultrasonic imaging apparatus, an ultrasonic probe having a transducer adapted to transmit and receive ultrasonic waves and a transmitter/receiver circuit for producing a transmission signal (also referred to as a driving pulse or a exciting pulse) and processing a received signal (an echo signal) from the transducer are coupled by means of a cable (conductive wire).
Since the transducer of the ultrasonic imaging apparatus operates to receive an ultrasonic reflected wave and convert it to an electric signal in a receive mode, the transducer can be regarded as a signal source when viewed from the body of apparatus. According to a general principle of the voltage transfer, lower output impedance of the signal source is better from the standpoint of signal loss. However, the output impedance of the transducer is relatively high and the received signal tends to attenuate due to the impedance of the conductive wire and the transmitter/receiver circuit, particularly parallel electrostatic capacitance. The amount of attenuation depends on the ratio of the impedance of the signal source to the impedance of the transmitter/receiver circuit.
Assume that the voltage of the signal source is V1, the impedance C of the cable and the transmitter/receiver circuit is C=Cp // Cc // Ctr and inductance L is null. Then a voltage V2 of the received signal will be given by EQU V2={Rtp/(Rp+Rtr+j.omega.C.multidot.Rp.multidot.Rtr)}.times.V1 (1)
where Cp is the electrostatic capacitance of the ultrasonic probe, Cc is the electrostatic capacitance of the cable, Ctr is the electrostatic capacitance of the body of the apparatus, Rp is the resistance of the probe, and Rtr is the resistance of the body of the apparatus. From equation (1) it will be understood that the larger the values of C, .omega., the lower the voltage V2 of the received signal. The equation also reveals that the higher the signal frequency used in the apparatus, the larger the amount of attenuation of the received signal.
For this reason, the following measures have conventionally be attempted against the attenuation of the received signal:
(1) To connect inductance L in parallel with the electrostatic capacitance C of the conductive wire and the transmitter/receiver circuit so as to cause parallel resonance at about the transducer center frequency of fo=1/(2.pi.LC). This will not cause the signal attenuation at about the resonance frequency.
(2) To connect an impedance transformer between the transducer and the conductive wire so as to increase the impedance seen at the transducer to the conductive wire and the transmitter/receiver circuit. This will decrease the signal attenuation.
(3) To decrease the electrostatic capacitance of the conductive wire and the transmitter/receiver circuit.
(4) To decrease the impedance of the transducer.
However, the above measures have the following problems.
In the case of measure (4), since the impedance of the transducer is determined by its physical size and material, the impedance cannot be set to a desired value. In the case of measure (3), since the electrostatic capacitance tends to increase because of the complication of the transmitter/receiver circuit due to its multichannel and high performance and the high-density version of the transmitter/receiver circuit due to its complication, it is difficult to decrease the electrostatic capacitance. Measure (2) is effective for a newly developed probe which is large in size. The measure cannot be applied to existing ultrasonic probes. Measure (1) is effective for use with an apparatus in which the electrostatic capacitance is determined, but cannot be used in common with another apparatus with different electrostatic capacitance because the value of the inductance L must be altered. Moreover, the value of the inductance L must be set for each channel, when each of channels has different electrostatic capacitance. Particularly when the electrostatic capacitance becomes large, the value of inductance L must be made small, in which case the Q value (sharpness) will become large and thus the effective frequency range will become narrow. AS a result, a sufficient signal will not be obtained.